a. Field of the Invention
This invention pertains to resonators for use at microwave frequencies. More particularly, this invention pertains to the use of shorted, slow-wave coplanar transmission lines as microwave resonators or filters and devices for coupling to such resonators and filters.
b. Description of the Prior Art
U.S. Pat. No. 5,777,532 ("532") discloses an interdigital, slow-wave coplanar transmission line consisting of two conducting strips that are located upon the surface of a substrate and that have interleaved conducting fingers that extend from the one side of each strip towards the opposite strip. The "532" patent also discloses a second, balanced configuration in which a central conducting strip is bounded on both sides by conducting grounds. Conducting fingers extend from the sides of the central conductor towards the grounds and interleave with conducting fingers extending from the grounds towards the central conductor. The capacitance between the interleaved fingers substantially slows the rate at which an electromagnetic wave propagates along the transmission line in comparison to the rate of propagation in the absence of such interleaved fingers. As a consequence a shorter piece of such transmission line provides a substantially greater time delay as compared with a coplanar transmission line that has no such interleaved fingers.
A section of ordinary coaxial transmission line that is short circuited at one end has been used for impedance matching purposes, and in some instances has been used as a frequency filter, or resonator, at MF, HF, VHF and in some instances at LHF frequencies. The input impedance at the input end of the transmission line opposite to the end that is short-circuited, exhibits a high impedance when the transmission line has a length of one-quarter wave, or an odd integer multiple thereof, and exhibits a low impedance when the transmission line has a length of one-half wave or an integer multiple thereof. At microwave frequencies, waveguides have also been used in a similar manner for the same purposes. However, at microwave frequencies, attempts to use short-circuited coplanar transmission lines as resonators and filters have been severely limited because higher order modes, other than the TEM mode(transverse electric and magnetic field mode) begin to propagate at frequencies that are not far removed from the microwave frequencies at which the shorted section of coplanar transmission line resonates.